1. Field of the Inventions
This invention relates to the field of imaging, particularly to the field of digital imaging, to the field of scanning images with a collimated beam of radiation and more particularly to the correction of lens-induced aberrations in the scanning process.
2. Background of the Art
The use of digital imaging has become extremely successful in a wide area of technical fields. In addition to the use of light beams (e.g., collimated light beams such as lasers) to directly image such diverse media as photographic elements, photothermographic elements, thermal mass transfer carriers, ablation-based systems, and the like), lasers are quite efficient in the rapid imaging of intermediate substrates that can be subsequently treated to provide images. Such intermediate imaging systems include, by way of non-limiting examples, electrography, photoelectrography, photolithography, photoconductive systems, and the like.
The use of lasers in digital systems is particularly effective as the lasers themselves generate spots (the incident areas of the laser beams), and as the lasers are effectively pulsed, the spots can be used to form pixels. Each spot can theoretically be a pixel (the smallest consistent repeating element of a picture, i.e., picture element), but ordinarily, the spots are small and numerous spots are used to create a pixel on even fairly high resolution images.
The accuracy of spot placement (and hence the resolution of pixel placement) is dependent upon a number of technical features, including especially the accuracy of the physical control of the positioning of the radiation beam on the focal plane. Among the technical features that contribute to this accuracy are software, mechanical components (moving the beam or the receiving surface), stability of mechanical components (avoiding vibration, reduced wear and the like), and the accuracy and quality of the beam of radiation itself. The last feature, beam quality, is particularly affected by the quality of the lens used in directing the laser beam. For the sake of simplicity in the discussion of this invention, the entire projection system may hereinafter be referred to as the “lens.” However, the term lens should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a lens. The position of a second element traversed by the projection beam relative to a first element traversed by the projection beam will for simplicity hereinafter be referred to as “downstream” of or “upstream” of said first element. In this context, the expression “downstream” indicates that a displacement from the first element to the second element is a displacement along the direction of propagation of the projection beam; similarly, “upstream” indicates that a displacement from the first element to the second element is a displacement opposite to the direction of propagation of the projection beam. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as printing, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake, development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an integrated circuit (IC). Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
There is a desire to integrate an ever-increasing number of electronic components in an IC. To realize this it is necessary to decrease the size of the components and therefore to increase the resolution of the projection system, so that increasingly smaller details, or line widths, can be projected on a target portion of the substrate. For the projection system this means that the projection system and the lens elements used in the projection system must comply with very stringent quality requirements. Despite the great care taken during the manufacturing of lens elements and the projection system they both may still suffer from wave front aberrations, such as, for example, displacement, defocus, astigmatism, coma and spherical aberration across an image field projected with the projection system onto a target portion of the substrate. Said aberrations are important sources of variations of the imaged line widths occurring across the image field. It is important that the imaged line widths at different points within the image field are constant. If the line width variation is large, the substrate on which the image field is projected may be rejected during a quality inspection of the substrate. Using techniques such as phase-shifting masks, or off-axis illumination, the influence of wave front aberrations on the imaged line widths may further increase.
During manufacture of a lens element it is advantageous to measure the wave front aberrations of the lens element and to use the measured results to tune the aberrations in this element or even to reject this element if the quality is not sufficient. When lens elements are put together to form the projection system it may again be necessary to measure the wave front aberrations of the projection system. These measurements may be used to adjust the position of certain lens elements in the projection system in order to minimize wave front aberrations of the projection system.
After the projection system has been built into a lithographic projection apparatus, the wave front aberrations may be measured again. Moreover, since wave front aberrations are variable in time in a projection system, for instance, due to deterioration of the lens material or lens heating effects local heating of the lens material), it may be necessary to measure the aberrations at certain instants in time during operation of the apparatus and to adjust certain movable lens elements accordingly to minimize wave front aberrations. The short time scale, on which lens-heating effects may occur, may require measuring the wave front aberrations frequently.
The use of an interferometric measurement system for in-situ measurement of wave front aberrations of the projection system of a lithographic projection apparatus is described in P. Venkataraman, et al., “Aberrations of steppers using Phase Shifting Point Diffraction Interferometry”, in Optical Microlithography XIII, J. Progler, Editor, Proceedings of SPIE Vol. 4000, 1245-1249 (2000). A Phase Shifting Point Diffraction Interferometry method and a corresponding system are disclosed in P. P. Naulleau et al., U.S. Pat. No. 6,100,978, issued Aug. 8, 2000, incorporated herein by reference. The Phase Shifting Point Diffraction Interferometry measurement method and corresponding measurement system will be referred to hereinafter as the PSPDI method and PSPDI system, respectively. The disclosed PSPDI systems feature the following elements, mentioned here in the order wherein these elements are traversed by the projection beam: a first pinhole in an object plane; a grating (with a one dimensional periodic structure of lines and spaces) between the object plane and the projection system, for generating by diffraction a test beam and a reference beam; the projection system, and a set of two pinholes comprising a window pinhole (traversed by the test beam) and a reference pinhole (traversed by the reference beam, and acting as a spatial filter for generating an unaberrated reference beam) in the plane that is optically conjugate to the object plane. The test beam and the reference beam generate an interference fringe pattern on a detector surface downstream of the set of two pinholes. This interference fringe pattern carries information on wave front aberrations. The grating, generally embodied as a grating pattern on a plane surface of a carrier substrate, acts as a beamsplitter; the grating shall be located downstream of said object plane such as to provide sufficient lateral separation of the areas traversed by the reference beam and the test beam in the plane that is optically conjugate to the object plane. Further, the grating is movable in a direction perpendicular to the direction of propagation of the projection beam such as to provide “phase shifting” (as explained below) of the interference fringe pattern with respect to a coordinate system associated with the detector surface, as needed for measuring aberrations.
Said phase shifting of the interference fringe pattern involves shifting the interference fringe pattern with respect to said coordinate system. For an explanation of “phase shifting” in relation to interferometry see, for example, D. Malacara, “Optical Shop Testing”, John Wiley & Sons, Inc., New York, second edition. Movement of an optical element (such as, for example, a grating) to provide phase shifting will be referred to hereinafter as “phase stepping”. A finite movement of an optical element (such as, for example, a grating) to provide a finite phase shift of said interference fringe pattern will be referred to hereinafter as a “phase step”.
An embodiment of a PSPDI system in a lithographic projection apparatus comprises, besides the support structure for supporting patterning structure and the substrate table for holding a substrate, one or more dedicated, movable support structures for supporting the grating and/or for moving the grating into and out of the projection beam and/or for phase stepping the grating. Incorporation of these one or more dedicated support structures into the lithographic projection apparatus leads to added mechanical complexity and increased costs of manufacturing the lithographic projection apparatus. Further, as explained above, in a PSPDI system each individual beam (the test beam and the reference beam) impinging on the detector has traversed two pinholes, one pinhole upstream of the projection system, and one pinhole downstream of the projection system. This circumstance typical for a PSPDI system poses a limitation to the amount of radiation that may reach the detector, and hence, to the sensitivity of the measurement system. U.S. Pat. No. 6,650,699 addresses these problems in a lithographic projection apparatus including an illumination system; a support structure for holding a mask; a substrate table for holding a substrate; a projection system for projecting a pattern onto a target portion of the substrate; and an interferometric measurement system for measuring wave front aberrations of the projection system, characterized in that the interferometric measurement system including: a grating, featuring a grating pattern in a grating plane, the grating being movable into and out of the projection beam, such that the grating plane is substantially coincident with the object plane; a pinhole, featuring a pinhole pattern in a pinhole plane and arranged in a pinhole plate, the pinhole being movable into and out of the projection beam, such that the pinhole plane is substantially coincident with a plane downstream of the projection system and optically conjugate to the object plane, and a detector with a detector surface substantially coincident with a detection plane, the detection plane located downstream of the pinhole at a location where a spatial distribution of the electric field amplitude of the projection beam is substantially a Fourier transformation of a spatial distribution of the electric field amplitude of the projection beam in the pinhole plane.
U.S. Pat. No. 6,323,959 (Toyama et al.) describes an image processor that receives color image data and converts them to image data of cyan, magenta, yellow and black aberration correction, and various image data processing is performed on the image data after the color aberration necessary for image forming. The received color image data are subjected to color correction in order to improve the precision of image processing on a color document image. The image data processing includes, for example, a specified color document is decided, automatic color selection and magnification change. The system operates by using a color image sensor which outputs image signals in correspondence to a plurality of colors; an optical system which focuses an image optically onto said color image sensor; a color aberration corrector which corrects image signals received from said color image sensor to cancel a displacement of focal point of the optical images of the colors caused by color aberration of said optical system; and a magnification changer which changes a magnification power of the image signals corrected by said color aberration corrector. This is primarily an attempt to correct chroma aberration induced by an optical lens, a phase difference among R, G and B is larger at positions closer to the ends of a document in the main scan direction.
When printing color images with an electrophotographic imaging system, there are usually four different monochrome colors that need to be combined into one full color print. The subtractive primary colors normally used in this process are cyan, magenta and yellow. Because the “process” black formed by combining all three primary colors is not usually an acceptable black, a fourth color (black) is normally used and the full color printing machine prints in cyan, magenta, yellow and black (C, M, Y and K).
In most color printing systems, a tandem approach is used whereby four separately produced monochrome images are combined (hopefully in good registration) to form the full color print. Each of the four primary images may be formed with the same scanning device (a 4-pass system) or each may be formed using a separate scanner (4 scanners for 4 color images) in which case the system is called a 1-pass system.
When different scanners are used (as in the 1-pass system) care must be taken to ensure that all four scanners are alike in their imaging character. Specifically, the focused spot of each of the four scanners must travel along the scan path in an identical (or near identical) manner which means that the velocity of the spot is constant in all scanners or that the velocity variations of the spot traveling along the scan are identical for all four scanners.
For example, if the yellow monochrome image is recorded with a perfectly linear scanner (spot velocity constant along the scan) but the cyan image is recorded with a scanner whose spot velocity is not so constant, then there will be areas in the color print where the yellow image and the cyan image are mis-registered and associated areas where the yellow image and the cyan image are well registered. It is of interest, therefore, to know about a scanner's spot velocity along its scan line and, more specifically, to know exactly where each pixel will be written in the presence of scanner non-linearities.
One commonly used method for determining scanner linearity behavior is to write an image onto a receptor material (such as paper) and then measure the image to determine if all the pixels along the scan line are properly placed or if some of the pixels may not have been placed correctly. These kinds of measurements are tedious and require an entire printing system to produce the measurable print.
The use of high quality glass refractive lenses has been required to minimize the aberrations in systems that require high quality, high resolution images. These lenses are very expensive, especially for mass produced imaging systems. However, when plastic lenses are used, the degree of aberration produced is extremely significant. It is not unusual to find pixel displacements of 5-10 pixel dimensions in actual images. This is unacceptable for high quality images, but has been used to make systems cost effective in markets that have become cost competitive, as in electrophotographic imaging systems.